17.3  Halophytes and adaptation to salt

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Halophytes are adapted to saline soils, and occur naturally in environments ranging from maritime estuaries to remnant salt lakes in arid zones (see Case studies 17.1 and 17.2). Vascular halophytes are widely distributed among many families of flowering plants, including grasses, shrubs and trees. Well-known inland halophytes of Australasia include saltbushes (Atriplex spp.) and samphires (Halosarcia spp.) while mangroves (diverse genera) are characteristic of coastal wetlands.

Halophytes commonly require some salt (soil solution c. 10–50 mM NaCl) to reach maximum growth, and a few halo-phytes, for example Atriplex nummularia (old man saltbush), grow best around 100 mM NaCl (Figure 17.18). Many halophytes can grow in full strength or even concentrated seawater (mangroves); and some unicellular organisms (Dunaliella salina) can withstand saturated salt (c. 5.5 M NaCl).



Figure 17.18 Growth responses for a range of halophytes, compared to one of the most salt tolerant cultivated plants, barley. Atriplex nummularia (old man saltbush) is one of the most tolerant inland species known (data from Greenway 1968). The samphire species is Halosarcia pergranulata subsp. pergranulata (P.G. Wilson) ‘black-seeded samphire’, a common samphire in southwestern Western Australia (data from Short and Colmer, unpublished). Compare these halophytes with species of agricultural importance in Figure 17.3.

Halosarcia (coll. samphires) is a genus comprising several species and subspecies, all of which are perennial shrubs or subshrubs with succulent stems (Figure 17.27). The genus is found only in countries bordering the Indian Ocean and is well represented within Australia (Wilson 1980). Halosarcia spp. inhabit a range of saline environments that are also prone to waterlogging, such as coastal saltmarshes, mud flats, estuaries, margins of inland salt lakes, brackish seepages and saline clay pans. Due to their high degree of tolerance to salinity and waterlogging, Halosarcia spp. have been used in the revegetation of salt-affected areas in the Western Australian wheatbelt.

Curiously, only dicotyledonous halophytes respond positively to salinity. Monocotyledonous species show little or no growth stimulation with increasing salinity (e.g. Kallar grass in Figure 17.18). Nevertheless, most halophytes accumu-late large amounts of NaCl that contribute substantially to plant dry mass. For example, salts contributed about 10% of the dry mass of A. nummularia grown near its optimum salinity of 200 mM NaCl. The salt concentration in the leaf tissues of halophytes can be greater than 500 mM (up to 1 M; see Table 17.4) which is well above maximum concentrations commonly found in non-halophyte species. Enzymes cannot function in such a high NaCl concentration, but adaptive features of halophytes allow metabolism to continue in that environment (discussed later).



Figure 17.19 Germination response of saltbush and wheat. Germination responses for three saltbush (Atriplex) species to increasing concentrations of NaCl were identical to those from iso-osmotic concentrations of mannitol. Germination response for a typical Australian bread wheat, Egret, confirms substantial NaCl tolerance during germination. Similar responses were obtained for barley cultivars. (Based on Osmond et al. 1980 for Atriplex, and Rana Munns, unpublished, for cereal germination)

Paradoxically, germinating seeds and young seedlings of halophytes are sensitive to salt, despite their extraordinary tolerance as adults. Seed germination is poor at NaCl con-centrations of 300 mM, or about 1.5 MPa of osmotic pressure. Indeed, seeds of halophytes are no more tolerant of salt at germination than are those of many cultivated species. For example, saltbush and wheat show a similar tolerance of germination to NaCl (Figure 17.19) Inhibition of seed ger-mi-nation by high salinity is osmotic in nature because iso-osmotic concentrations of mannitol exert similar effects on germination of Atriplex spp.

Some halophytes have evolved with reproductive features that forestall salt inhibition of germination, and ensure that seed shed and subsequent germination coincides with periods of low salinity. For example, many Atriplex species carry their seeds in bracts that contain high levels of NaCl. When hydrated, the salt concentration in these bracts is over 500 mM, which inhibits germination. However, this salt is leached from the bracts by substantial rain that will also generate a favourable seed bed. Accordingly, shed seeds then take up water and germinate.

Wetland halophytes such as mangroves carry additional adaptive features that enable propagation and establishment in waterlogged saline sediments where even heavy rain is insufficient to dilute soil salt. Mangroves do not discharge their seeds to the soil surface, but retain them so that the seed germinates on the parent plant. Such germination, or ‘vivipary’, refers to continued development of an embryo, with little or no dormant phase. When viviparous seedlings detach from their parent tree, their radicles have already elongated to form a root with cellular structures to exclude salt.